For the commercial utilization of solar energy by photovoltaic current generation in the terrestrial sector, silicon is at the present the only suitable semiconductor material both for economic and for ecological reasons. Silicon originating from semiconductor silicon, of which the production process is known from semiconductor technology has hitherto been almost exclusively used.
Whereas material costs are of minor importance in the semiconductor industry, they are crucially important in photovoltaics because the electrical output of a solar cell is proportional to its area and hence to the amount of material involved.
Photovoltaics can contribute towards solving terrestrial energy problems when solar cells capable of competing with conventional energy sources in price and efficiency have been successfully developed. Accordingly, a basic prerequisite for the utilization of solar energy on a large scale is an economical process for the production of silicon which satisfies the solar silicon requirements.
Various processes have been proposed for the production of solar silicon, including:
1. The reduction of high-purity quartz sand with high-purity carbon (DE-A 3 013 319),
2. The purification of metallurgical silicon (DE-A 2 623 413), the reduction of SiO.sub.2 with aluminium (EP-A 0 029 182),
3. The reduction of SiF.sub.4 or SiCl.sub.4 with sodium (Mater. Res. Bull. 16(4), 437 (1981); DE-A 2 800 254),
4. The reduction of SiCl.sub.4 with zinc (U.S. Pat. No. 3,012,862) and
5. The reduction of SiCl.sub.4 with aluminium at 360.degree. C. (P. Pascal, Nouveau traite de chimie minerale, Vol. VIII, No. 2, Silicon, page 275).
Economically, these processes are not satisfactory for the production of a silicon for solar cells on an industrial scale. EP-A-0 123 100 describes a process in which solid aluminium is reacted with Si-halides at 500.degree. to 660.degree. C. A pure silicon suitable for solar cells is economically obtained by this process.
The reaction of liquid aluminium with SiCl.sub.4 at 750.degree. to 1000.degree. C. is described by Yoshizawa et al (Kagyo kagaku Zasshi 64 (1961), 1347 to 1350)Silicon tetrachloride is passed over a bath of liquid aluminium at the reaction temperature. At most 50% by weight silicon with adhering aluminium is obtained. The silicon was isolated by dissolving the residual aluminium in acid. This method is not practicable for an economic process. Since the diffusion of dissolved silicon in the molten aluminium (D=10.sup.-5 cm.sup.2 /S)is the speed-determining factor in the reaction described by Yoshizawa et al, the reaction times are relatively long. A complete conversion of the aluminium in an economically reasonable time does not occur.
The reaction is largely determined by the temperature, the atmosphere and the SiCl.sub.4 throughput. The disadvantages mentioned above persist despite the described optimization.
Accordingly, the object of the present invention was to provide an economical process without any of the disadvantages described above.